Deep Well Casing Cathodic Protection Requirements

Abstract

ABSTRACT Failures of well casings as a result of external corrosion on bothcathodically protected and unprotected wells in the South Swan Hills Unit havebeen investigated. Previous testing indicated that an instantaneously appliedcurrent of 30.0 amperes would not provide adequate protection throughout thecorrosive Wabamun D-1 water-bearing zone which exists throughout the unit atdepths ranging between 1,676 m (5,500 ft) and 2,195 m (7,200 ft). A four-well pilot project was initiated in early 1977 to determine if 30.0amperes and polarization would provide adequate cathodic protection throughoutthe D-1 zone. The results of the pilot project indicated that cathodicprotection with 30. 0 amperes and polarization is possible throughout thecorrosive zone and that cathodic protection currents can be successfullyprojected below high-resistance zones. Introduction The South Swan Hills Unit is located approximately 225 km (140 miles)northwest of Edmonton, Alberta (Fig. 1). The field was discovered in 1961 andwas placed on production the following year. The total daily Unit production isapproximately 9,540 m3 (60,000 barrels) of oil and 9,540m3 (60,000 barrels) of water from 161 producing wells. The Unitcontains a total of 228 wells. A typical downhole diagram of a South Swan Hill Unit well is depicted in Figure 2. The average total depth of the wells in the Unit is 2,469 m (8,100ft) and the average calculated cement top is 1,280 m (4,200 ft). The wellsproduce from the Beaverhill A and B formations. A total of 13 leaks attributed to external corrosion have been experiencedin Unit wells since 1966. As shown in Figure 2, the leaks have occurred in twodistinct ranges: 8 between 894 m (2,900 ft) and 1,340 m (4,400 ft) on wellsdrilled with low-pH gyp starch mud, and 5 between 1,890 m (6,200 ft) 2,286 m(7,500 ft) on both non-gyp starch and gyp starch mud wells. The corrosionoccurring over the lower level is attributed to the water-bearing Wabamun D-1zone. Previous testing had indicated that cathodic protection over the D-1 zonemay not be possible in the South Swan Hills field.